This invention relates to an intake air filter for the internal combustion engine of a motor vehicle which comprises a housing with a raw air inlet for unfiltered air and a clean air outlet for filtered air and two annular filter inserts disposed concentrically one inside the other inside the housing through which air from the raw air inlet flows radially to the clean air outlet.
German Utility Model No. DE-G 73-21,762 discloses an intake air filter for internal combustion engines, compressors and other air-aspirating machines, which comprises two filter inserts disposed concentrically one inside the other. The filter inserts are fastened by a central threaded bolt anchored in the clean air intake. The bolt extends through openings in the cover plates and clamps the filter inserts axially against the housing parts by means of nuts. Abutments are provided on the bolt to hold the filter inserts in proper position, and the cover plates rest on the abutments.
This type of fastening arrangement requires a very high accuracy in the position of the abutment surfaces on which the filter inserts are supported in the housing. Furthermore, similar high accuracy in the length of the filter inserts also is required in order to assure reliable sealing of the end face of the filter insert that lies adjacent the clean air intake. In filter units which must be replaced frequently, this requirement for high accuracy leads to unreasonably high costs.
Another disadvantage of the threaded bolt arrangement is that the bolt must be fastened inside the clean air intake. This may be possible, for example, by spot welding if the housing is made of sheet metal. If the housing is made of plastic and the clean air intake is integrated into the housing, a bolt can only be fastened to the housing at the high cost of providing reinforcing ribs, metal inserts, etc. Even if a rather expensive construction is used, there is a danger that the creep of the plastic over a long period of time will lead to a decrease in the holding forces acting on the filter insert, so that the seal between the raw air chamber and the clean air chamber within the filter is threatened by loosening of the filter insert.